1. Field of the Invention
The present invention relates generally to equipment for cleaning printing presses.
2. Description of the Prior Art
Over the last few decades, printing presses have been considerably improved, with many functions improved through automation. The peripheral equipment used in combination with printing presses have also evolved considerably. In contrast, a group of functions, though directly linked to the printing quality and process, but apparently of secondary importance, have been somewhat neglected by the manufacturers of printing presses.
In order to facilitate and understanding of the present invention, several functions necessary for the operation of an offset printing press (and particularly those linked to the invention) are described generally below. The offset printing mode has been selected to illustrate the invention, since it requires a certain number of supplementary functions compared to the other printing modes (such as, for example, typographic printing, flexographic printing, and others).
These functions can be defined generally as: (1) cleaning the inking units; (2) cleaning the printing blankets; (3) cleaning the printing plates; (4) cleaning the printing cylinders; and (5) cleaning the damping rollers.
The inking unit have to be cleaned at least once a day for presses using inks of the same color, and must be cleaned each time an ink of a different color is used. In the later case, the cleaning must be much more thorough to prevent traces of the ink from the preceding run from remaining on the surface of the inking and distributing rollers and altering the color of the subsequent ink. Changing inks can be especially problematic when a light colored ink follows a darker colored ink. In that case, it is important to clean the press several times over, re-inking it between each cleaning with white ink or the following ink, to absorb and eliminate traces of the previous ink.
The printing blankets also must be frequently cleaned. In addition, fibers and foreign particles stuck to the surface of the printing blankets must be removed. These fibers come from the sheets or paper web that is compressed between the blanket and pressure cylinders during the printing process. The number of cleaning operations needed depends upon several parameters such as, for example, the load and nature of the ink, the type of paper to be printed, the size of the run to be carried out. These cleaning operations can be performed, depending upon these parameters, on average about six to fifteen times a day, and sometimes much more.
The printing plates also require cleaning and frequent maintenance to keep good printing definition. After each prolonged stop the printing plates must be coated with an Arabic gum or similar product diluted in water to prevent their oxidization, which would very quickly render them useless. This operation consists of depositing a film of this dilution on all the plate areas which have been previously well cleaned, to isolate it from the oxygen in the air.
The printing cylinders also need to be cleaned since, during the printing or cleaning of the other elements, they are often dirty. This dirt will stain the back of the printed material without directly affecting the quality of the printing, unless there are large deposits of hardened ink on the printing cylinder.
The conventional damping device uses dampening rollers which are covered with flannel sleeves which require daily cleaning when one ink is used but also require cleaning when changes of ink occur, since particles of ink are emulsified by the dampening water rinse and catch on the surface of the sleeves. Cleaning is usually accomplished with a brush and detergent. This cleaning operation is tedious and involves decoupling of the damping rollers from the press.
After lengthy stops or changes to the printing speed, variations in the supply of dampening water cannot be avoided. Time is therefore spent making adjustments to obtain uniform damping and a stable balance between ink and water.
Sometimes, fibers from the covering of the damp-form rollers are deposited on the blanket or on the plate. These cause the formation of spots or marks which affect the printing quality. In recent years, all sorts of dampening devices have been devised to avoid these limitations. These devices are all based on the concept of adding an alcohol low in toxicity, such as propanol, to the dampening waters to alter the surface tension of the dampening water to allow a sleeveless roller to transfer a thin uniform film of water.
Another, more recent improvement has been made by a water treatment and refrigeration unit which automatically takes into account and adjusts the alcohol and Ph content of the dampening water, the alcohol and Ph dosing the temperature of refrigeration, and filtration of the dampening water. In these water treatment and refrigeration units, treated water is circulated by means of a pump to establish a permanent circulation between the treatment and refrigeration unit and the press. Dampening water is returned from the dampening trays of the press to the unit where it is treated and homogenized before being reinjected into the circuit. This manner of preceding has major advantages including: (1) improved printing quality; (2) consistent printing quality without regard to room temperature; (3) a slower evaporation of the water on the plate; (4) improvement in the quality of water film requiring much less water (about 30% less) and thus enabling the shine of the inks to be improved and the risks of staining to be lessened.
It is probable that these treatment and refrigeration units will come into general use very quickly and replace conventional systems.
At present, the great majority of printing presses are equipped with devices enabling them to clean, imperfectly and in an amateurish fashion, all components of the inking unit including the inking distribution rollers and cylinders, with the exception of the ink fountain which is generally cleaned manually. This method of cleaning is old and in wide use.
Briefly, it consists of applying a rubber blade to the median line of one of the driven rollers of the inking unit after a solvent has been applied to the unit. This action has the effect of detaching the film of the ink from the roller. The removed ink is collected in a tray which is coupled to the blade. Each roller and cylinder is cleaned one after the other. The end result is a group of imperfectly cleaned rollers which accumulate ink in the tiny pores of their surfaces. Ink molecules will harden and eventually totally transform the state of the microporous surface into a shining slope unsuitable for retaining, distributing, and forming a required film of ink. The desired thickness of which is about ten to twenty microns and become unsuitable for distributing a fine and even layer of ink on the printing parts.
This method of cleaning with a blade presents a certain number of shortcomings and inadequacies, including: (1) While in use the blade becomes covered, on both sides, by a layer of ink diluted by the solvent. After scraping, the blade is removed from the cylinder. The layer of ink separates into two parts, one remains stuck to the cylinder and spreads itself immediately over all the rollers. (2) The blade, even when properly maintained and cleaned, becomes polluted with unwanted ink which then mixes with the succeeding washing inks. (3) The general condition of the blade has a direct influence on the final result. (4) The ink and its solvent form an agglomerate which dries and becomes firmly fixed on the tray and on both sides of the blade. The unit has to be scoured by hand periodically. Besides being tedious, the process is wasteful of the printer's time since tinting operations are suspended for cleaning. As a result, the printer puts off cleaning as long as possible.
Certain equipment for automatically or semi-automatically cleaning blanket cylinders has been available in recent years. These items of equipment involve different techniques which use a variety of mechanical means such as blades, brushes, and rollers in combination with detergents mixed in water to remove the surface ink and bits of paper fixed on the blanket. Such equipment has, until recent years, been in the form of attachments for presses. They are sometimes very sophisticated and controlled completely be electronic means enabling variable cleaning cycles to be programmed depending on a certain number of criteria. The most recent models pick-up the residual products by means of a spongy strip unrolled from one reel and automatically rewound during the absorption of the residual products. This procedure allows for a quick, clean and simple recovery of the residual products. The major press manufacturers are sporadically beginning to equip certain of their models with less sophisticated blanket washers. However, the cleaning and gumming of the plates is still carried out by hand on all offset presses.
It should be noted that all the functions cited above are an integral part of the printing process. Every professional printer realizes the vital importance of the surface conditions of his inking rollers and printing blankets. He or she knows that this surface condition is one of the essential elements to keep in order to retain rapid quality printing. Unfortunately, nowadays, the printer finds himself or herself confronted with contradictory demands. On one hand, intensive production is required. On the other hand, methodical maintenance of the printing press is required. Often, the printer finds himself or herself neglecting the daily maintenance of the printing press.